Hydrogeological and hydrogeochemical examination of the geothermal area of the lower Sakarya basin

The aim of this study was to investigate the hydrogeological and hydrogeochemical properties of the Lower Sakarya geothermal area. The metamorphism of the Sakarya continent and the Rhodope-Pontid (Istanbul-Zonguldak Zone) fragment, which are formed as a result of the approaching of each other, and the North Anatolian Fault formed in the region reveal the evolutionary geology of the study area. The lithological units in the study area are classified as West Pontide zone, Armutlu-Almacık-Arkotdağı zone, Sakarya zone and cover units. The heat source is thought to be magmatic intrusions due to the North Anatolian Fault. The reservoir rocks consist of calc schist and marble levels of the Sultaniye metamorphics, limestone and marble blocks of the Abant formation. The clayish schist levels of the metamorphic rocks form the cover rock. The level of shales and mudstones of the Abant formation showing the flysch character forms the cover rock. Çaycuma, Yığlıca and Örencik formations are generally impermeable. The impermeable levels form the cover rock-whereas the granular parts are secondary reservoirs. In this study, water samples were collected from seven different locations and were made cation, anion and isotope analyses. The waters show generally Na-Cl, Ca-Cl and Na-Ca-Cl types. The dominant cations and anions vary regionally in the study area. The geothermal waters sampled are classified as environmental water according to the Cl-SO₃-HCO₄ triangle diagram. According to the hot Giggenbach diagram, hot waters in the study area generally fall into the class of immature waters except for one sample that falls into the partially equilibrated water class. Silica geothermometry result show that the reservoir temperatures of the thermal waters generally vary between 18 °C and 152 °C. The reservoir temperatures calculated by cation geothermometers range from 32 °C to 38 °C. Comparing the measured outlet temperatures of the geothermal wells and thermal springs having between 20 °C- 89 °C it is on opinion that silica geothermometers give more reliable results than cation geothermometers. δ2H values of thermal waters rang from -81.47 to -71.63, while δ18O values are between -11.97 and -7.05. According to the δ18O- δ2H graph, it was determined that the oxygen content of the hot water in the region enriched due to the rock-water interaction. Calculations of mineral saturation indices of calcite, dolomite, aragonite, gypsum, anhydrite, quartz and chalcedony minerals in hot water samples were evaluated. The common features of all samples are that the dolomite, calcite and aragonite minerals were oversaturated and that the quartz, gypsum and anhydrite minerals were under saturated.